1. Field of the Invention
The present invention relates to a process for the preparation of exocellular biopolymers that exhibit a thickening effect in aqueous media.
2. Statement of Related Art
European Patent Application No. 0 058 364 and U.S. Pat. No. 4,618,582 relate to a process for the preparation of Xanthomonas biopolymers by aerobic culture of microorganisms of the genus Xanthomonas in an aqueous nutrient medium which is characterized in that the microorganisms are grown in a water-in-oil emulsion (W/O emulsion) which is stable under fermentation conditions. In this process, the microorganism may first be grown in an oil-free aqueous nutrient medium, a W/O emulsion subsequently formed with addition of the oil phase and fermentation continued with the W/O emulsion intact. The process is preferably carried out in the presence of W/O emulsifiers, the W/O emulsifier best being initially dissolved in the oil phase and the resulting solution subsequently mixed with the aqueous phase.
European Patent application No. 0 098 473 relates to an extended version of this process in which microorganism strains which generally form exocellular biopolymers having a thickening effect in the aqueous nutrient medium, but which do not belong to the genus Xanthomonas, are grown in W/O-emulsions of the type in question. The microorganism strains mentioned include, for example, gram-positive or gram-negative bacterial strains, fungi or algae. A corresponding teaching is given in U.S. Pat. No. 4,352,882 where particular significance is again attributed to the culture of Xanthomonas strains.
The use of W/O emulsions of the above type in the aerobic culture of, for example, xanthan-forming Xanthomonas strains affords important advantages in several respects. For example, the W/O-emulsion technique eases or eliminates the difficulties encountered in the aqueous fermentation phase of the process through the increasing degree of thickening initiated by the polysaccharide accumulating as fermentation product. Where purely aqueous nutrient media are used, yields of only 2 to 3% by weight xanthan (dry matter) may be regarded as a good result which can only be obtained by adopting special measures involving high energy consumption. By changing over to culture of the polysaccharide-forming microorganism strains in the aqueous disperse phase of W/O emulsions, the viscosity of the liquid phase in the fermenter under fermentation conditions is reduced because the viscosity of the system as a whole is primarily determined by the homogeneous oil phase. It has also been found that, by changing over to W/O emulsion systems of the above type, it is possible to obtain improved xanthan yields in the disperse aqueous phase compared with working with a purely aqueous culture medium.
European Patent Application No. 0 135 133 relates to a process for reducing the viscosity of fermentation broths by addition of a halogenated hydrocarbon and formation of corresponding W/O emulsions. In this process, non-toxic, highly halogenated, more especially perhalogenated, aliphatic hydrocarbons which are liquid under the fermentation conditions are used as the halogenated hydrocarbon phases. No emulsifiers are used. This teaching is based on the discovery that, in the previously described processes for the formation of xanthan in W/O-emulsions, the viscosity of the medium as a whole may not only not be reduced, but even increased. This danger is said to exist in particular where emulsifiers are used.